Elucidation of the mechanism of carboxy ester cleavage promoted by a Cu(ii) alkoxide complex of a tripodal ligand (N3OX)

Abstract

A solution complexation study by potentiometric titration of the new Cu(II) complexes of the tripodal ligand 2-[bis(2-aminoethyl)amino]ethanol (L) has revealed that with coordination to Cu(II), the hydroxyl group of L exhibits a rather low pK_a value of 8.5 to give Cu(II) alkoxide, which is also confirmed by solution ES-MS. Release of the second hydrogen atom from the coordinated water occurs at pK_a = 10.6 at 25 °C. X-Ray diffraction analysis of [CuL(Cl)](ClO₄) revealed that Cu(II) adopts a trigonal-bipyramidal geometry, different from all the previously reported N₃OX–copper complexes. The CuL-promoted hydrolysis of 4-nitrophenyl acetate (NA) showed a second-order rate constant of 0.043 M⁻¹ s⁻¹ and further study of the hydrolysis mechanism provided evidence for the preference of a transesterification rather than a hydrolysis path. Additionally, an ab initio Hartree–Fock study strongly suggested that the Cu(II) alkoxide will be generated prior to Cu(II) hydroxide due to the energy preference for the former, with the energy difference estimated to be 55.77 kJ mol⁻¹, so the Cu(II) coordinated alcohol will exhibit a lower pK_a than Cu(II) coordinated water. Mulliken population analysis showed that the HOMO oxygen lone pair energy for Cu(II) alkoxide is 0.148 eV higher than that of Cu(II) hydroxide, according to which the oxygen of Cu(II) alkoxide is expected to be more nucleophilic.